1. Field of the Invention
This invention is concerned with a scanning optical system capable of forming a scanning light beam which moves at a high speed and in a broad range by the use of a polygonal mirror rotating in a predetermined direction. More particularly, the invention relates to a scanning optical system having means for providing an extremely low variation in the amount of light of the scanning light beam with rotation of the polygonal mirror.
In various apparatuses such as, for instance, recording devices for printing high speed patterns and character informations from electrical computers, etc., high speed read-out devices from hologram arrays, detection devices for surface defects of an object to be detected, and so forth, there have been great demands for a light beam scanning optical system which is operable at a high speed, compact in its construction, and sufficiently stable and precise in its operations.
2. Description of the Prior Art
A known scanning optical system which causes the light beam to scan on an object to be detected, and which includes a rotary polygonal mirror uses a method for increasing the scanning speed of the light beam in which the rotary polygonal mirror is rotated at a high speed, or another method, in which the number of reflection surface on the rotary polygonal mirror is increased.
However, an increase in the rotational speed of the polygonal mirror would be apprehensively liable to invite increase in frictional resistance with air to necessitate the driving mechanism to exert more power as well as to bring about undesirable noise. Also, this increase in the rotational speed results in unsatisfactory conditions of the device such as, for example, it requires an increased size for the structure, and reduces the stability and preciseness of operations, and so on.
Further, when the number of reflection surfaces of the rotary polygonal mirror is increased without changing the size of the reflection surface itself, the diameter of the rotary polygonal mirror becomes larger with a consequent increase in required power of the driving mechanism, generation of undesirable noise, or lack in compactness in its construction as well as inferior stability and preciseness in its operations.
Furthermore, when the number of the reflection surfaces is increased without changing the diameter of the rotary polygonal mirror, i.e. the area of each constituent reflection surface is reduced, and when a light beam of the same diameter is projected thereonto, there inevitably occurs such disadvantage that the angle of deflection of the light beam, within which the energy quantity of the light beam does not vary, becomes smaller than in the case where the area of the reflection surface remains unchanged. In other words, the period of ineffective scanning operation increases, and the scanning efficiency lowers accordingly. This phenomenon occurs because the ratio of the effective reflection region of a single reflection surface to the total area of the reflection surfaces on the rotary polygonal mirror has become reduced as the result of the relative reduction in area of the single reflection surface in comparison with the largeness in cross-section of the light beam. (The term "effective reflection region" as used herein is meant by the region wherein the light beam projected on one of the reflection surfaces is totally reflected at this particular reflection surface. In case the light beam enters into an edge region of one reflection surface, one part of this incident light beam is also projected onto an adjacent reflection surface and causes a decrease in the required light amount of the scanning light beam. The so-called "eclipse" of the light beam results thereby.)
In the following, explanations will be made as to an improved method of increasing the scanning speed of the light beam in consideration of the abovementioned defects.
A method which can be contemplated at first is one in which the light beam is converged on the reflection surface of the rotary polygonal mirror by the use of a converging optical system, i.e., a method wherein the cross-section of the light beam is reduced in correspondence to the smallness of the reflection surface. This method, however, has such disadvantage that the light beam once converged by the converging optical system is again diverged with the consequence that it assumes a wider cross-section at the position of the object to be scanned. In view of the fact that, in the print-out device for pattern and character informations output from electronic computers, for example, the photosensitive body should be scanned with a light beam in the form of a spot, so that such divergent light beam as mentioned above is of no use for the scanning purpose. If such divergent light beam is to be converged to a spot, a converging lens needs be disposed in the light path for this divergent light beam, whereupon the scanning of the object per se becomes impossible. The reason for this inability is that, since the abovementioned light beam diverges from substantially one point (a point at a position where the reflection surface is present), and, at the same time, moves angularly on this point as the center, the abovementioned converging lens acts to converge the scanning light beam and simultaneously direct the scanning light beam entirely onto the abovementioned single conjugate spot with respect to the converging lens.
Also, there can be contemplated another method, wherein sufficiently thin parallel light beams are projected onto the surface of the rotary polygonal mirror where the scanning light beam is reflected. In this method, however, as the resolution of the scanning optical system (i.e., number of resolving point on the surface to be scanned) is determined by a product of a diameter of the incident light beam onto the abovementioned reflection surface and a range of angle, within which the light beam moves angularly, after it is reflected, the diameter of the incident light beam is naturally limited, so that it is not feasible to make the diameter of the incident light beam excessively thin.
In the Japanese Patent Publication No. 49-16824 (published Aug. 16, 1969), there is disclosed a method for removing the abovementioned inconvenience when the area of the reflection surface of the rotary polygonal mirror is reduced. According to this method, the light beam is made to be projected onto the reflection surface of the rotary polygonal mirror by vibrating the light beam with a sinusoidal wave, saw-tooth wave, or the like wave forms in synchronism with rotation of this rotary polygonal mirror. Although this method successfully prevents a part of the light beam from projecting into adjacent reflection surface during a single required scanning period, it still is not free from the defect such that as the light beam reciprocatingly vibrates, there inevitably occurs an idle period for the scanning operation. That is, even when the subsequent reflection surface of the polygonal mirror arrives at the reflecting position to enable the light beam to move for the scanning upon completion of one scanning period, the required scanning operation can not be performed during the time instant in which the incident light beam is caused to return to the starting position, and it is virtually impossible to reduce this idle period. In order to shorten this idle period, the speed for the return motion of the abovementioned light beam should be made quicker than the speed for the advancing motion thereof. For this purpose, however, it is mandatory to utilize a superprecise vibrating optical system having a remarkably high responsive capability such as a saw-tooth vibrating mirror having an extremely sharp trailing characteristic.
Consequently, when the abovementioned method is applied in carrying out an efficient and high speed scanning operation, the performance of the abovementioned vibrating optical system makes it difficult to sufficiently shorten the idle period for the scanning.